It is necessary to save power in portable devices and devices which are not permanently connected to a power supply in order to increase the service life. This also applies to chip cards which are used, for example, in mobile telephones. It is likewise necessary to save power in contactless chip cards which are supplied with power via an electromagnetic field provided by a so-called chip card reader. Since the field strength is standardized on the basis of any possible interactions with adjacent devices, the current which can be obtained from the field for the chip card is also limited.
The increasing miniaturization of integrated circuits which are also used in chip cards is associated with leakage currents which cannot be ignored. This problem will be further aggravated in future as miniaturization advances.
In order to save power, provision has hitherto been made in circuits for individual functional blocks to be switched on and off as required. The functional blocks which are not immediately needed to execute the next program steps are switched off. In conventional chip structures containing a processor, a memory and, if appropriate, additional functional blocks, the operating system coordinates the operations of switching the individual functional blocks on and off.
The concept of switching off functional blocks which are not needed can also be applied to memory arrangements. Memory arrangements generally comprise a plurality of memory blocks. A memory block comprises a plurality of memory locations at which a respective data word can be stored. Each memory location is assigned a unique physical address which is used to access a memory location within a memory block for the purpose of reading and writing. The address decoder which has an input for applying an address assigns a memory location to the logical address applied, with the result that the same memory location is always written to or read from during write or read access operations with the same address applied on the input side. The corresponding data word is preferably read in and out via a second input for data words, said second input, for its part, not being important, however, for the further considerations.
In the case of previous power-saving memory arrangements which are usually in the form of random access memories, a memory controller switches the memory blocks on and off as required. The power supply for the relevant memory block is switched on before an access operation and is switched off again after the end of the access operation. The memory controller is a separate functional block which is controlled by an operating system in the same manner as the further functional blocks.
The disadvantage of the memory controller is that it requires additional power. The power consumption of a chip arrangement is therefore reduced through the use of a memory controller only when the power saving for the controlled memory arrangement is greater than the power consumption of the memory controller. This is the case only when the memory arrangement which is controlled by the memory controller is of an appropriate size. In this case, switching off a large number of memory blocks, or large memory blocks which consume a lot of power, as required offsets the power consumption of the memory controller.
Chip cards are distinguished by a relatively small memory. Memory sizes of 8 and 16 kilobytes are already regarded as being large for use in chip cards. Therefore, it is not worthwhile for chip cards to use a memory controller. This would merely lead to a higher power consumption since the power consumption of the memory controller would exceed the possible power saving of the memory arrangement.